The courtyard at the Infinity residential complex has a distinctive design and is open to the public, though few people linger.

Photo: Liz Hafalia, The Chronicle

The courtyard at the Infinity residential complex has a distinctive...

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The roof terrace at One Kearny is exactly that: a snug oasis in the sky with penthouse like panoramas west on Market Street and south to the bay.

Photo: Liz Hafalia, The Chronicle

The roof terrace at One Kearny is exactly that: a snug oasis in the...

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The 11th floor terrace of One Kearny provides a unique glimpse of the regal mansard roof of the 1906 landmark to which it is attached

Photo: Liz Hafalia / The Chronicle

The 11th floor terrace of One Kearny provides a unique glimpse of...

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The 11th floor Skygarden in the Federal Building has a remarkable view of South of Market, but there's no sign announcing it on 7th Street and there's a security checkpoint at the building's entrance.

Photo: Liz Hafalia, The Chronicle

The 11th floor Skygarden in the Federal Building has a remarkable...

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Except for the cozy lounge furniture, there's not much reason to visit the public terraces at the Intercontinental Hotel at 5th and Howard streets.

Photo: Liz Hafalia / The Chronicle

Except for the cozy lounge furniture, there's not much reason to...

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The public open space included with Millennium Tower is stylish, but most passers-by are unlikely to notice the dim high-ceilinged room.

Photo: Liz Hafalia, The Chronicle

The public open space included with Millennium Tower is stylish,...

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The lesson at 555 Mission St.? When you provide sun and outdoor space and a range of seating options, people like Steve Golebiewski (left, middle) and Scott Hanson (right, middle), both from San Francisco, are likely to put it to use.

Photo: Liz Hafalia, The Chronicle

The lesson at 555 Mission St.? When you provide sun and outdoor...

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Someone finds a quiet spot in the Skygarden at the Federal Building.

Photo: Liz Hafalia, The Chronicle

Someone finds a quiet spot in the Skygarden at the Federal Building.

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The public open space in the enclosed plaza at Millennium Tower.

Photo: Liz Hafalia, The Chronicle

The public open space in the enclosed plaza at Millennium Tower.

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The view from 1 Kearny’s roof terrace, one of San Francisco’s rooftop public spaces that aren’t always easy to find.

The Roof Terrace at One Kearny shows why we're lucky that San Francisco requires downtown developers to provide space in their projects that is accessible to the public at large.

It also is a case study in why the generation-old guidelines must be improved.

The snug terrace opened this winter as a comfortable perch that delivers the best free views downtown, whether it's the panoramic vista up Market Street to Twin Peaks or the adjacent red-tile mansard roof from 1906 that you can almost touch. But the only exterior hint that the terrace exists is a see-through sign etched into the glass at knee level by the front door. Once inside, a guard requires you to sign in before going farther.

Now, as a new wave of downtown development begins to gain momentum, San Francisco planners and decision-makers should make a worthy program better. Simple remedies to the current rules would increase awareness of good spaces and focus resources more effectively.

The provision of privately owned public open spaces is governed by the city's 1985 downtown plan. The formula "to meet the needs of downtown workers, residents and visitors" requires 1 square foot of public space per 50 square feet of office space or hotels.

At least 15 such spaces have been created since then because of the program. In addition, at least two recent projects not covered by the downtown plan include distinctive publicly accessible spaces: the San Francisco Federal Building with its three-story "sky garden" cut into the 18-story tower, and an expansive landscaped passage between the clover-shaped towers of the Infinity condominium complex.

Unique solution deserves to be seen

At One Kearny, the developer was required to provide roughly 2,500 square feet of space for the public - no easy feat, given that the project consists of a mid-block addition to a modest 1906 bank tower already fused to a 1964 wing.

The solution: pull back the addition's 11th floor to tuck in a terrace that also maintains views from the west of the 1906 landmark's regal mansard roof.

The result is unique, a vantage point of the sort that until now was available only to penthouse dwellers or corner-office executives. The space itself is amply outfitted with benches and planters.

The problem, again, is knowing that it exists.

The 1985 plan states that when public spaces are located within or on top of buildings, "their availability should be marked visibly at street level." But because the guidelines are so vague, it's easy to fulfill their letter but not their spirit.

That's true of One Kearny's hideaway. By placing the sign at knee level - and making it less than 5 inches wide - the likelihood of outsiders finding their way to the roof is almost nil.

At another recent space, the enclosed plaza included as part of the Millennium Tower, the exterior sign is brushed metal. But at 6 inches square, it's too easy to miss.

Compare this with the signs required for similar private-but-public spaces in New York City. The city's planning code requires signs to be "12 inches square in dimension and dark green or black in color with a highly contrasting background," with "lettering at least two inches in height stating 'OPEN TO PUBLIC.' "

And don't try tucking them out of sight: "The entry plaque shall be mounted on a wall or a permanent free-standing post ... with its center five feet above the elevation of the nearest walkable pavement."

Here in San Francisco, the 23 "parklets" that overtook parking spaces come with black signs 5 by 11 inches that don't mince words: Thick white letters proclaim "PUBLIC PARKLET" and in the line below "ALL SEATING IS OPEN TO THE PUBLIC."

Either of these models is preferable to the current approved city design, a tasteful but too demure depiction of a tree surrounding the words "PUBLIC OPEN SPACE" in small letters. It's urbane, not assertive, and the latter is what makes people stop.

In-lieu fees where spaces don't exist

Another issue involves not signage but whether some spaces should be there at all.

At Millennium Tower, for instance, the open space requirement was prompted when one portion of the building early on was considered for office space or a hotel. But the towers turned out all-residential, so the "public" space leads not to a mixed-use lobby but a dark wall of glass.

This issue is addressed under the district plan now proposed for the area around First and Mission streets. Developers of new towers in some cases would be allowed to satisfy their open-space requirement with an in-lieu fee. It's an option that makes more sense than insisting on spaces that then sit empty more often than not.

There are other challenges to tackle before San Francisco's semipublic spaces reach their full potential. But the first challenge is simply for people to know they exist. Once a constituency exists, issues like maintenance and openness will fall into line.